As the amount of information increases in recent years, there are growing demands for a recording medium capable of writing and retrieving a large amount of data at high speed and in high density. There are growing expectations that the optical disks Will meet this demand.
There are two types of optical disks: a write-once type that allows the user to record data only once, and a rewritable type that allows the user to record and erase data as many times as they wish. Examples of the rewritable optical disk include a magnetooptical recording medium that utilizes a magneto-optical effect and a phase-change type recording medium that utilizes a change in reflectance accompanying a reversible crystal state change.
The principle of recording an optical disk involves applying a recording power to a recording layer to raise the temperature of that layer to or above a predetermined critical temperature to cause a physical or chemical change for data recording. This principle applies to all of the following media: a write-once medium utilizing pitting or deformation, an magnetooptical medium utilizing a magnetic reversal at the vicinity of the Curie point, and a phase change medium utilizing a phase transition between amorphous and crystal states of the recording layer.
Further, taking advantage of the 1-beam-overwrite ability (erasing and writing at the same time) of the phase change recording medium, rewritable compact disks compatible with CDs and DVDs (CD-ReWritable and CD-RW) and rewritable DVDs have been developed.
Almost all of these optical recording media in recent years employ a mark length recording method, which is suited for increasing the recording density.
The mark length recording is a method that records data by changing both the lengths of marks and the lengths of spaces. Compared with a mark position recording method which changes only the lengths of the spaces, this method is more suited to increasing the recording density and can increase the recording density by as much as 1.5 times. However, to retrieve data accurately makes the detection of the time length of the mark stringent, thus requiring precise control of the shape of mark edges. Further, there is another difficulty that a plurality of kinds of marks with different lengths, from short marks to long marks, need to be formed.
In the following descriptions, the spatial length of a mark is referred to as a mark length and a time length of the mark as a mark time length. When a reference clock period is determined, the mark length and the mark time length have a one-to-one correspondence.
In the mark length recording, when writing an nT mark (a mark having a mark time length of nT where T is a reference clock period of data and n is a natural number), simply radiating a recording power of square wave with the time length of nT or with the length finely adjusted will result in the front and rear ends of each mark differing in temperature distribution, which in turn causes the rear end portion in particular to accumulate heat and widen, forming an mark with an asymmetric geometry. This raises difficulties in precisely controlling the mark length and suppressing variations of the mark edge.
To uniformly shape the marks, from short marks to long marks, various means have been employed, such as division of recording pulses and use of off pulses. For example, the following techniques have been adopted in the phase change media.
That is, a recording pulse is divided to adjust the geometry of an amorphous mark (JP-A 62-259229, JP-A 63-266632). This approach is also utilized in the write-once medium that is not overwritten. Further, an off pulse is widely employed as a mark shape compensation means (JP-A 63-22439, etc.)
Other proposed methods include one which deliberately dull a trailing edge of the recording pulse to adjust the mark length and the mark time length (JP-A 7-37252); one which shifts a recording pulse radiation time (JP-A 8-287465); one which, in a multipulse recording method, differentiates a value of bias power during the mark writing operation from that during the space writing operation or erasing operation (JP-A 7-37251); and one which controls a cooling time according to a linear velocity (JP-A 9-7176).
The recording method based on the above pulse division approach is also used in the magnetooptical recording medium and the write-once type optical recording medium. In the magnetooptical and write-once type mediums, this approach aims to prevent heat from becoming localized. In the phase change medium, this approach has additional objective of preventing recrystallization.
Common examples of mark length modulation recording include a CD compatible medium using an EFM (Eight-Fourteen Modulation), a DVD compatible medium using an EFM+ modulation, a variation of 8-16 modulation, and a magnetooptical recording medium using a (1, 7)-RLL-NRZI (Ruu-Length Limited Non-Return to Zero Inverted) modulation. The EFM modulation provides 3T to 11T marks; the EFM+ modulation provides 3T to 14T marks; and the (1, 7)-RLL-NRZI modulation provides 2T to 8T marks. Of these, the EFM+ modulation and the (1, 7)-RLL-NRZI modulation are known as modulation methods for high-density mark length modulation recording.
As the recording pulse division scheme for the mark length modulation recording media such as CD, the following method is widely used.
That is, when a mark to be recorded has a time length of nT (T is a reference clock period and n is a natural number equal to or greater than 2), the time (n−η)T is divided intoα1T, β1T, α2T, β2T, . . . , αmT, βmT(where Σαi+Σβi=n−η; η is a real number from 0 to 2; m is a number satisfying m=m−k; and k is 1 or 2). In a time duration of αiT (1≦i≦m) as the recording pulse section, recording light with a recording power Pw is radiated. In a time duration of βiT (1≦i≦m) as the off pulse section, recording light with a bias power Pb, less than Pw, is radiated.
FIG. 2 is a schematic diagram showing a power pattern of the recording light used in this recording method. To form a mark of a length shown in FIG. 2(a), a pattern shown in FIG. 2(b) is used. When forming a mark that is mark-length-modulated to the length of nT (T is a reference clock period; and n is a mark length, an integer value, that can be taken in the mark length modulation recording), (n−η)T is divided into m=n−k (k is 1 or 2) recording pulses (in the case of FIG. 2(b), k=1 and i=0.5), and the individual recording pulse widths are set to αiT (1≦i≦m), each followed by the off pulse section of βiT (1≦i≦m). In the αiT (1≦i≦m) section during the recording, the recording light with the recording power Pw is radiated and, in the βiT (1≦i≦m) section, the bias power Pb (Pb<Pw) is radiated. At this time, to ensure that an accurate nT mark can be obtained during the detection of the mark length, Σαi+Σβi may be set slightly smaller than n, and the following setting is made: Σαi+Σβi=n−η (η is a real number in 0.0≦η≦2.0).
That is, in the conventional technique, when the recording light to be radiated to form an nT mark is divided, the recording pulse is divided into m pieces (m=n−k, where k is 1 or 2), m being obtained by uniformly subtracting k from n (as described in JP-A 9-282661), and then a predetermined number is subtracted from the number of divisions m of the recording pulse to control the mark time length accurately (in the following, such a pulse division scheme is called an “n−k division” scheme).
Generally, the reference clock period T decreases as the density or speed increases. For example, T decreases in the following cases.
(1) When the recording density is enhanced to increase the recording capacity:
As the mark length and the mark time length are reduced, the density increases. In this case, a clock frequency needs to be increased to reduce the reference clock period T.
(2) When the recording linear velocity is increased to increase a data transfer rate:
In the high-speed recording of recordable CDs and DVDs, the clock frequency is increased to reduce the reference clock period T. In a CD-based medium such as a rewritable compact disk, for example, the reference clock period T during a ×1-speed operation (linear velocity is 1.2-1.4 m/s) is 231 nanoseconds; but during a ×10-speed operation the reference clock period T becomes very short, 23.1 nanoseconds. In the DVD-based medium, while the reference clock frequency T during a ×1-speed operation (3.5 m/s) is 38.2 nanoseconds, it is 19.1 ns during a ×2-speed operation.
As can be seen from the (1) and (2), in large-capacity optical disks and CDs and DVDs with high data transfer rates, the reference clock period T is very short. As a result, the recording pulse section αiT and the off pulse section βiT also tend to become short. Under these circumstances the following problems arise.
(Problem a)
The recording pulse section αiT may be too short for the rising/falling edge speed of radiated light, particularly a laser, to follow. A rise time is a time taken by the projected power of radiated light such as laser to reach a set value, and a fall time is a time taken by the projected power of the radiated light such as laser to fall from the set value to a complete off level. At present the rise and fall times take at least 2-3 nanoseconds respectively. Hence, when the pulse width is less than 15 ns, for example, the time it takes for the light to actually project a required power is a few nanoseconds. Further, when the pulse width is less than five nanoseconds, the projected power begins to fall before it reaches the set value, so that the temperature of the recording layer does not rise sufficiently, failing to produce a predetermined mark size. These issues of response speed limits of a signal source and a laser beam cannot be dealt with by making improvements on the wavelength of a light source, on the method of radiating light onto substrate/film surface, or on other recording methods.
(Problem b)
When the off pulse section βiT is narrow, the recording medium cannot take a sufficient time to cool down and the off pulse function (cooling speed control function) does not work although the off pulse section is provided, leaving heat to be accumulated in the rear end part of the mark, making it impossible to form the correct shape of the mark. This problem becomes more serious as the length of the mark increases.
This problem will be explained by taking a phase change medium as an example.
The currently available phase change medium typically takes crystal portions as an unrecorded state or erased state and amorphous portions as a recorded state. To form an amorphous mark involves radiating a laser onto a tiny area of the recording layer to melt that tiny portion and quickly cooling it to form an amorphous mark: When, for example, a long mark (a mark more than about 5T in length based on the EFM modulation recording for CD format) is formed using a rectangular waveform of recording power with no off pulse section at all, as shown in FIG. 3(a), then an amorphous mark with a narrow rear end is formed as shown in FIG. 3(b) and a distorted retrieve waveform is observed as shown in FIG. 3(c). This is because, in the rear part of the long mark in particular, heat is accumulated by heat diffusion from the front part enlarging the melted area in the rear part but the cooling speed deteriorates significantly allowing the melted area to recrystallize as it solidifies. This tendency becomes conspicuous as the linear velocity for recording decreases because the cooling speed of the recording layer becomes slower as the linear velocity decreases.
Conversely, if the cooling speed is so high as to render recrystallization almost negligible, when a long mark is recorded, an amorphous mark with a thicker rear end is formed as shown in FIG. 3(d), producing a distorted retrieve waveform as shown in FIG. 3(e). This is explained as follows. In the rear end of the long mark in particular, heat is accumulated by heat diffusion from the front part enlarging the melted area in the rear part and the shape of the melted area is transformed into the shape of an amorphous mark relatively precisely because the cooling speed is kept relatively high over the entire area.
When a plurality of off pulse sections are not distributed and properly used over the entire mark length, recrystallization becomes conspicuous somewhere in the mark, as shown in FIG. 3(b) and 3(d) though in different degrees, preventing a good formation of an amorphous long mark and causing distortions in the retrieve waveform.
Inserting the off pulse sections makes sharp the temperature change over time of the recording layer ranging from the front end to the rear end of the long mark, preventing degradation of the mark due to recrystallization during recording.
However, as the reference clock frequency T becomes shorter because of increased density and speed as described above, the rapid cooling becomes difficult to achieve even with the off pulse sections provided in a conventional manner, resulting in the front half of the mark being recrystallized.
For example, when a mark with a time length of 4T is to be recorded on a CD-RW, a phase change type rewritable compact disk, by the conventional n−k division scheme (k=1), the following pulses are radiated during the process of forming the amorphous mark:α1T, β1T, α2T, β2T, α3T, β3T
Here, the starting end of the mark is melted by the application of the recording pulse α1T and then heat produced by the application of the subsequent recording pulses α2T, α3T conducts toward the front part of the mark. FIG. 4 is a schematic temperature history of the mark starting end, with FIG. 4(a) representing a case in which the linear velocity is low and FIG. 4(b) a case in which the linear velocity is high. In either case, three temperature rising processes due to α1T, α2T, α3T and three cooling processes due to β1T, β2T, β3T are observed.
In the case of low linear velocity, as shown in FIG. 4(a), there are sufficient cooling times at β1T, β2T, during each of which the temperature of the cooling layer can fall below the crystallization temperature. In the case of high linear velocity, however, because the reference clock period T decreases in inverse proportion to the linear velocity, the recording layer melted by the α1T is heated by the next α2T and further by α3T without cooling below the crystallization temperature range, as shown in FIG. 4(b). The time during which the recording layer stays in the crystallization temperature range is much longer for T4+T5+T6 of the high linear velocity than for T1+T2+T3 of the low linear velocity, so it is understood that the recrystallization is more likely to take place at the fast linear velocity. In an alloy with a composition close to a SbTe eutectic composition and used as a phase change recording layer, a crystal is likely to grow at the amorphous/crystal boundary and therefore recrystallization easily occurs outer area of the mark. Here, the low speed refers to less than about ×10-speed (T=less than 23.1 nanoseconds) and the high speed refers to about ×10-speed or more.
As described above, in the phase change medium, as the reference clock period T becomes short due to an increased density and speed, recrystallization is likely to occur with the conventional pulse division scheme, giving rise to a serious problem that a required degree of modulation fails to be generated at the central part of the long mark.
In the phase change medium in which an amorphous mark is recorded over a crystal area, although it is generally easy at high linear velocity to secure an enough cooling speed to form an amorphous solid, the crystallization time is difficult to secure. Hence, the phase change medium often employs a recording layer of a composition which tends to be easily crystallized, i.e., a recording layer of an easily recrystallizable composition. Therefore, it is important to increase the off pulse section to enhance the cooling effect, but during the high linear velocity the off pulse section becomes short to the contrary.
The similar problem is also encountered when the wavelength of a laser source is reduced or a numerical aperture is increased to reduce a beam diameter for enhancing the density of the phase change medium. For example, when a laser with a wavelength of 780 nm and a numerical aperture of NA=0.50 is changed to a laser with a wavelength of 400 nm and a numerical aperture of 0.65, the beam diameter is throttled to almost one-half. At this time, the energy distribution in the beam becomes steep so that the heated portion is easily cooled, allowing an amorphous mark to be formed easily. This however makes the recording layer more difficult to crystallize. In this case, too, it is necessary to increase the cooling effect.
The present invention has been accomplished to solve the aforementioned problems. It is an object of the invention to provide an optical recording method and an optical recording medium suited for the method, which can perform recording in a satisfactory manner even during a mark length recording using a short clock period suited for high density recording and high speed recording.